This invention relates to communications and has particular relationship to communications which for security and electronic counter-measure purposes has the capability of spread-spectrum operation. Such communications is disclosed in the parent application. In transmission, a carrier is modulated in accordance with the intelligence being communicated and in addition is phase-shift-key modulated (o or .pi. radians; that is by phase reversals) with a digital pseudo-noise (PN) code. Typically, as shown in the parent application, the code may be provided by a code generator in the form of a shift register comprised of a number of flip-flops feed-back coupled through an exclusive OR. The number of bits in the code is determined by the number, N, of flip-flops and is 2.sup.N -1. The frequency of the clock which clocks these number of bits will then determine the code repetition period. Typically, there may be 51 stages in the code generator and for this number of bits the period is about 17 years for a clock frequency of 5 mhz. The effect of the modulation with the code is to spread and transform the spectrum of the intelligence-modulated carrier. The envelope of the spread-frequency spectrum then has the form ##EQU1## The narrow frequency band of this spread spectrum containing the intelligence is not detectable even by receivers having positive signal-to-noise ratios which are not equipped with auto-correlation apparatus.
To derive the intelligence on reception, the PN code is auto-correlated with a replica code. The received carrier modulated by the intelligence plus a PN code, is phase-shift-key modulated by a PN code which is substantially the same as, and is maintained in synchronism with, the PN code of the transmitter. To maintain the synchronism, a periodic tracking reference oscillator or dither is impressed on the received carrier which is being auto-correlated. In accordance with the teachings of the parent application this dither is an amplitude modulation derived from the reference oscillator which is converted to a phase modulation. This phase modulation passes through the receiver network and its phase is compared with the phase of the signal from the reference oscillator to derive a signal for compensation or regulation of the PN coder of the auto-correlator via a tracking loop.
The invention of the parent application is applicable to RF output of different types; amplitude, angle phase, or frequency modulated or others. However, heretofore it has not been possible to apply the teaching of the parent application to communications equipment in which the receiver network includes a limiter which removes amplitude modulations.
It is an object of this invention to overcome the disability of the parent application and to provide communications having the facility for spread spectrum operation notwithstanding that the receiving units may or may not include limiters through which the correlated, intelligence-modulated carrier passes. It is another object of this invention to provide an adapter for converting conventional communications equipment, particularly of the frequency modulation or phase modulation type, in which the receiver may or may not have a limiter, to spread-spectrum operation. It should be pointed out that the receiver limiter does not preclude full operation of the technique.